This is the IRR of the loan's cashflow, after adding all
the extra costs
Usage
cft(amt, maturity, rate, up_fee = 0, per_fee = 0)
Arguments
amt
The amount of the loan
maturity
The maturity of the loan
rate
The loan rate, in effective rate
up_fee
The fee that the loan taker pays upfront
per_fee
The fee that the loan payer pays every
period
Details
It is assumed that the loan has monthly payments The CFT is
returned as an effective rate of periodicty equal to that
of the maturity and the rate The interest is calculated
over amt + fee